Definition

The term ‘hydrocephalus' is from two Greek words meaning ‘water in the head'. In fact, the ‘water' is cerebrospinal fluid (CSF), a clear liquid which is produced all the time in the cavities or ventricles inside the brain. It passes from one ventricle to the next (four in all) through narrow pathways, then circulates around the surface of the brain - a little also goes down the spinal cord - and is absorbed back into the bloodstream. The absorption takes place through specialised veins inside the skull which have a sieve-like surface. Though much slower than the circulation of the blood, the CSF is constantly being produced, circulated and reabsorbed.

Hydrocephalus can result when either too much CSF is produced (very rare), or when it is prevented from circulating or being reabsorbed - the two most common causes. As in these circumstances CSF is constantly produced but cannot get out, it accumulates and causes raised pressure inside the brain. The ventricles swell and the brain tissue is stretched and squashed. The skull bones in babies and young children are not fixed together as they are in later life, and the pressure causes the head to increase in size. However it is important to realise that hydrocephalus can also arise in older children and in adults, when the skull bones are fixed and the head cannot increase in size.